Japanese Patent Application Laid-Open Publication No. 2002-208807 and Japanese Patent Application Laid-Open Publication No. 2000-216605 disclose an example of a line transducer (a line transition element) that performs conversion between a microstrip line and a waveguide. FIG. 14 shows a first embodiment, and FIG. 15 shows a second embodiment, of Japanese Patent Application Laid-Open Publication No. 2002-208807. In this conventional technology, a microstrip line 210 and an external waveguide 212 are connected via a dielectric ridged waveguide 211. The line transducer in FIG. 14 includes a multilayer dielectric substrate 201b laminated on an external waveguide 212, a dielectric substrate 201a laminated above this, a ground conductor pattern 202 laminated on the undersurface of the dielectric substrate 201a, a strip conductor pattern 203 laminated on the top surface of the dielectric substrate 201a, waveguide-forming conductor patterns 204a, 204b provided on each surface of the multilayer conductor substrate 201b, ridge-forming conductor patterns 205a, 205b, a ground conductor pattern gap 206 provided on the ground conductor pattern 202, a conductor pattern gap 207 provided on the waveguide-forming conductor pattern 204b, a waveguide-forming via 208, and ridge-forming via 209. The strip conductor pattern 203 and ground conductor pattern 202 disposed on the top and bottom of the dielectric substrate 201a form the microstrip line 210. The dielectric substrate 201a, multilayer dielectric substrate 201b, ground conductor pattern 202, waveguide-forming conductor patterns 204a, 204b, ridge-forming conductor patterns 205a, 205b, and waveguide-forming via 208 and ridge-forming via 209, form the dielectric ridged waveguide 211.
The line transducer of FIG. 15 includes a multilayer dielectric substrate 201b laminated on an external waveguide 212, a dielectric substrate 201a laminated above this, a ground conductor pattern 202 laminated on the undersurface of the dielectric substrate 201a, a strip conductor pattern 203 laminated on the top surface of the dielectric substrate 201a, waveguide-forming conductor patterns 204a, 204b provided on each surface of the multilayer conductor substrate 201b, ridge-forming conductor patterns 205a, 205b, a ground conductor pattern gap 206 provided on the ground conductor pattern 202, a conductor pattern gap 207 provided on the waveguide-forming conductor pattern 204b, a waveguide-forming via 208.
The line transducer of FIG. 15 further includes ridge-forming vias 209a, 209b, these ridge-forming vias 209a, 209b forming the dielectric ridged waveguide 211, and functioning as a two-step impedance transformer.
In the example disclosed in Japanese Patent Application Laid-Open Publication No. 2000-216605, a line transducer between a microstrip line (radiofrequency line conductor) and the waveguide is a “ridged waveguide” formed in a step-like shape wherein a connecting line conductor is disposed parallel in the same transmission direction as that of the microstrip line, and the gap between upper and lower main conductor layers in the waveguide line of the connecting part is made narrow.
The standard waveguide which is designed from the viewpoint of suppressing conductor loss has a characteristic impedance of several hundred Ω. In order to directly connect to the standard waveguide, it will be assumed that the characteristic impedance of an external waveguide (e.g., the external waveguide 212 in FIG. 24) is equal to the characteristic impedance of the standard waveguide such that the reflection loss is low. On the other hand, the characteristic impedance of a microstrip line is often designed to be 50Ω so as to match the IC in the measurement system or the RF (Radio Frequency) circuit. To connect a transmission line of such different characteristic impedance, a λ/4 transducer is used.
When a transmission line having a characteristic impedance of Z1 is connected to a transmission line having a characteristic impedance of Z2, the λ/4 transducer is a line of length λ/4 having a characteristic impedance of Z3 (:Z3=√(Z1*Z2)). The magnitude relationship between the characteristic impedances is given by inequality (1):Z2<Z3<Z1  (1)
In the example of Japanese Patent Application Laid-Open Publication No. 2002-208807, it is seen that if the characteristic impedance of the external waveguide 212 is Z1, and the characteristic impedance of the microstrip line 210 is Z2, the characteristic impedance of the dielectric ridged waveguide 211 is Z3, which is an intermediate value between Z1 and Z2. As a means of decreasing the characteristic impedance of the dielectric ridged waveguide 211 to less than that of the external waveguide, the shortest side of the rectangular cross-section of the waveguide can simply be shortened, but since a ridged waveguide having a transmission mode approximating that of the microstrip line is ideal, this is what is used in the conventional technology.
However, if the characteristic impedance ratio between the external waveguide 212 and microstrip line 210 is large, the reflection loss increases, and it is difficult to suppress the line transition loss to a minimum. In the example of Japanese Patent Application Laid-Open Publication No. 2002-208807, in order to resolve this problem, the lengths of the ridge-forming vias 209a, 209b forming the dielectric ridged waveguide 211 are respectively arranged to be λ/4, and the dielectric ridged waveguide 211 is split as shown in FIG. 15. Thus, plural dielectric ridged waveguides having different characteristics impedances were disposed in columns between the external waveguide 212 and microstrip line 210, and by suppressing the characteristic impedance ratio, the line transition loss was suppressed.
One subject should be taken into consideration in using waveguides of this structure is that of reducing the line loss due to the conversion of characteristic impedances and transmission modes between the microstrip lines and the waveguides.
In the conventional technology, characteristic impedance matching between these lines is achieved using a λ/4 matching box, which is a millimeter waveband impedance matching means, to reduce the assembly loss. In another technique, to connect a transmission line having a large characteristic impedance difference, a line transducer is formed using plural λ/4 transducers to reduce the reflection loss, as shown in FIG. 15.
FIG. 9 shows the reflection loss of a line transducer using an ordinary λ/4 transducer. Here, a low impedance waveguide and a 380Ω standard waveguide are connected using a λ/4 transducer. The diagram shows the results of a simulation using four characteristic impedances, i.e., 40Ω, 108Ω, 158Ω, and 203Ω. It is seen that for a connection with a 203Ω waveguide having a characteristic impedance ratio of about 2, the reflection loss is −34 dB, and with 40Ω having a characteristic impedance ratio of about 9, the reflection loss worsens to −11 dB.
For example, for a 50Ω microstrip line with a 380Ω standard waveguide, since the characteristic impedance ratio is about 8, the characteristic impedance ratio must be reduced by using two or more λ/4 transducers having a characteristic impedance ratio of about 3≈380/108 to keep the reflection loss at −20 dB or below. If Z1=3*Z2, the characteristic impedance Z3 of the λ/4 transducer is given by equation (2):Z3√{square root over (Z1×Z2)}=√{square root over (3)}·Z2  (2)
Therefore, the characteristic impedance of the λ/4 transducer which is first connected to the microstrip line, is that of an 86Ω waveguide having a characteristic impedance of √3 times 50Ω, i.e., 86Ω.
However, for connecting between a microstrip line and a waveguide, the waveguide structure is not sufficient in itself to achieve loss reduction only by characteristic impedance matching of the line.